Streptococcus species, of which a large variety causes infections in domestic animals and man, are often grouped according to Lancefield's groups. Typing according to Lancefield occurs on the basis of serological determinants or antigens that are, among others, present in the capsule of the bacterium, and allows for only an approximate determination. Often, bacteria from different groups show cross-reactivity with each other, while other Streptococci cannot be assigned a group-determinant at all. Within groups, further differentiation is often possible on the basis of serotyping. These serotypes further contribute to the large antigenic variability of Streptococci, a fact that creates an array of difficulties within diagnosis of and vaccination against Streptococcal infections.
Lancefield group A Streptococcus species (Group A Streptococci “GAS,” Streptococcus pyogenes) are common in children, causing nasopharyngeal infections and complications thereof. Among animals, cattle are especially susceptible to GAS infection which can cause mastitis.
Group A streptococci are the etiologic agents of streptococcal pharyngitis and impetigo, two of the most common bacterial infections in children, as well as a variety of less common, but potentially life-threatening, infections including soft tissue infections, bacteremia, and pneumonia. In addition, GAS are uniquely associated with the post-infectious autoimmune syndromes of acute rheumatic fever and post streptococcal glomerulonephritis.
Several recent reports suggest that the incidence of both serious infections due to GAS and acute rheumatic fever has increased during the past decade, focusing renewed interest on defining the attributes or virulence factors of the organism that may play a role in the pathogenesis of these diseases.
GAS produce several surface components and extracellular products that may be important in virulence. The major surface protein, M protein, has been studied in the most detail and has been convincingly shown to play a role in both virulence and immunity. Isolates rich in M protein are able to grow in human blood, a property thought to reflect the capacity of N protein to interfere with phagocytosis, and these isolates tend to be virulent in experimental animals.
Lancefield group B Streptococcus (“GBS”) are most often seen in cattle, causing mastitis; however, human infants are susceptible as well, often with fatal consequences. Group B streptococci (GBS) constitute a major cause of bacterial sepsis and meningitis among human neonates born in the United States and Western Europe and are emerging as significant neonatal pathogens in developing countries as well.
It is estimated that GBS strains are responsible for 10,000 to 15,000 cases of invasive infection in neonates in the United States alone. Despite advances in early diagnosis and treatment, neonatal sepsis due to GBS continues to carry a mortality rate of 15 to 20%. In addition, survivors of GBS meningitis have 30 to 50% incidence of long-term neurologic sequelae. Over the past two decades, increasing recognition of GBS as an important pathogen for human infants has generated renewed interest in defining the bacterial and host factors important in virulence of GBS and in the immune response to GBS infection.
Particular attention has focused on the capsular polysaccharide as the predominant surface antigen of the organisms. In a modification of the system originally developed by Rebecca Lancefield, GBS strains are serotyped on the basis of antigenic differences in their capsular polysaccharides and the presence or absence of serologically defined C proteins. While GBS isolated from non-human sources often lack a serologically detectable capsular, a large majority of strains associated with neonatal infection belong to one of four major capsular serotypes, 1a, 1b, II or III. The capsular polysaccharide forms the outermost layer around the exterior of the bacterial cell, superficial to the cell wall. The capsule is distinct from the cell wall-associated group B carbohydrate. It has been suggested that the presence of sialic acid, in the capsule of bacteria that causes meningitis, is important for allowing these bacteria to breach the blood-brain barrier. Indeed, in S. agalactiae, sialic acid has been shown to be critical for the virulence function of the type III capsule. The capsule of S. suis serotype is composed of glucose, galactose, N-acetylglucosamine, rhamnose and sialic acid.
The group B polysaccharide, in contrast to the type-specific capsule, is present on all GBS strains and is the basis for serogrouping the organisms into Lancefield's group B. Early studies by Lancefield and co-workers showed that antibodies raised in rabbits against whole GBS organisms protected mice against challenge with strains of homologous capsular type, demonstrating the central role of the capsular polysaccharide as a protective antigen. Studies in the 1970s by Baker and Kasper demonstrated that cord blood of human infants with type III GBS sepsis uniformly had low or undetectable levels of antibodies directed against the type III capsule, suggesting that a deficiency of anticapsular antibody was a key factor in susceptibility of human neonates to GBS disease.
Lancefield group C infections, such as those with S. equi, S. zooepidemicus, S. dysgalactiae, and others, are mainly seen in horses, cattle and pigs, but can also cross the species barrier to humans. Lancefield group D (S. bovis) infections are found in all mammals and some birds, sometimes resulting in endocarditis or septicemia.
Lancefield groups E, G, L, P, U and V (S. porcinus, S. canis, S. dysgalactiae) are found in various hosts, causing neonatal infections, nasopharyngeal infections or mastitis.
Within Lancefield groups R, S and T (and with ungrouped types), Streptococcus suis is an important cause of meningitis, septicemia, arthritis and sudden death in young pigs (4, 46). Incidentally, it can also cause meningitis in man (1). S. suis strains are usually identified and classified by their morphological, biochemical and serological characteristics (58, 59, 46). Serological classification is based on the presence of specific antigenic polysaccharides. So far, 35 different serotypes have been described (9, 56, 14). In several European countries, S. suis serotype 2 is the most prevalent type isolated from diseased pigs, followed by serotypes 9 and 1. Serological typing of S. suis is performed using different types of agglutination tests. In these tests, isolated and biochemically characterized S. suis cells are agglutinated with a panel of 35 specific sera. These methods are very laborious and time-consuming.
Little is known about the pathogenesis of the disease caused by S. suis, let alone about its various serotypes such as type 2. Various bacterial components, such as extracellular and cell-membrane associated proteins, fimbriae, hemagglutinins, and hemolysis, have been suggested as virulence factors (9, 10, 11, 15, 16, 47, 49). However, the precise role of these protein components in the pathogenesis of the disease remains unclear (37). It is well known that the polysaccharide capsule of various Streptococci and other gram-positive bacteria plays an important role in pathogenesis (3, 6, 35, 51, 52). The capsule enables these microorganisms to resist phagocytosis and is therefore regarded as an important virulence factor. Recently, a role of the capsule of S. suis in the pathogenesis was suggested as well (5). However, the structure, organization and function of the genes responsible for capsule polysaccharide synthesis (“cps”) in S. suis is unknown. Within S. suis, serotype 1 and 2 strains can differ in virulence for pigs (41, 45, 49). Some type 1 and 2 strains are virulent, other strains are not. Because both virulent and non-virulent strains of serotype 1 and 2 strains are fully encapsulated, it may even be that the capsule is not a relevant factor required for virulence.
Attempts to control S. suis infections or disease are still hampered by the lack of knowledge about the epidemiology of the disease and the lack of effective vaccines and sensitive diagnostics. It is well known and generally accepted that the polysaccharide capsule of various Streptococci and other gram-positive bacteria plays an important role in pathogenesis. The capsule enables these microorganisms to resist phagocytosis and is therefore regarded as an important virulence factor.
Compared to encapsulated S. suis strains, non-encapsulated S. suis strains are phagocytosed by murine polymorphonuclear leucocytes to a greater degree. Moreover, an increase in thickness of capsule was noted for in vivo grown virulent strains while no increase was observed for avirulent strains. Therefore, these data again demonstrate the role of the capsule in the pathogenesis for S. suis as well.
Ungrouped Streptoccus species, such as S. mutans, causing caries in humans, S. uberis, causing mastitis in cattle, and S. pneumonia, causing major infections in humans, and Enterococcus faecilalis and E. faecium, further contribute to the large group of Streptococci.
Streptococcus pneumoniae (the pneumococcus) is a human pathogen causing invasive diseases, such as pneumonia, bacteremia, and meningitis. Despite the availability of antibiotics, pneumococcal infections remain common and can still be fatal, especially in high-risk groups, such as young children and elderly people. Particularly in developing countries, many children under the age of five years die each year from pneumococcal pneumonia. S. pneumoniae is also the leading cause of otitis media and sinusitis. These infections are less serious, but nevertheless incur substantial medical costs, especially when leading to complications, such as permanent deafness. The normal ecological niche of the pneumococcus is the nasopharynx of man. The entire human population is colonized by the pneumococcus at one time or another, and at a given time, up to 60% of individuals may be carriers. Nasopharyngeal carriage of pneumococci by man is often accompanied by the development of protection against infection by the same serotype. Most infections do not occur after prolonged carriage but follow exposure to recently acquired strains. Many bacteria contain surface polysaccharides that act as a protective layer against the environment. Surface polysaccharides of pathogenic bacteria usually make the bacteria resistant to the defense mechanisms of the host, for example, the lytic action of serum or phagocytosis. In this respect, the serotype-specific capsular polysaccharide (“CP”) of Streptococcus pneumoniae, is an important virulence factor. Unencapsulated strains are avirulent, and antibodies directed against the CP are protective. Protection is serotype specific; each serotype has its own, specific CP structure. Ninety different capsular serotypes have been identified. Currently, CPs of 23 serotypes are included in a vaccine.
Vaccines directed against Streptococcus infections typically aim to utilize an immune response directed against the polysaccharide capsule of the various Streptococcus species, especially since the capsule is considered a primary virulence factor for these bacteria. During infection, the capsule provides resistance against phagocytosis and thus protects the bacteria from the immune system of the host, and from elimination by macrophages and neutrophils.
The capsule particularly confers the bacterium resistance to complement-mediated opsonophagocytosis. In addition, some bacteria express capsular polysaccharides (CPs) that mimic host molecules, thereby avoiding the immune system of the host. Also, even when the bacteria have been phagocytosed, intracellular killing is hampered by the presence of a capsule.
It is generally thought that the bacterium will get recognized by the immune system through the anticapsular-antibodies or serum-factors bound to its capsule and will, through opsonization, get phagocytosed and killed only when the host has antibodies or other serum factors directed against capsule antigens.
However, these antibodies are serotype-specific, and will often only confer protection against only one of the many serotypes known within a group of Streptococci.
For example, current commercially available S. suis vaccines, which are generally based on whole-cell-bacterial preparations, or on capsule-enriched fractions of S. suis, confer only limited protection against heterologous strains. Also, the current pneumococcal vaccine that was licensed in the United States in 1983, consists of purified CPs of 23 pneumococcal serotypes whereas at least 90 CP types exist.
The composition of this pneumococcal vaccine was based, on the frequency of the occurrence of disease isolates in the US and cross-reactivity between various serotypes. Although this vaccine protects healthy adults against infections caused by serotypes included in the vaccine, it fails to raise a protective immune response in infants younger than 18 months and it is less effective in elderly people. In addition, the vaccine confers only limited protection in patients with immunodeficiencies and hematology malignancies.